Chem. prob. please help pH prob?

A buffer containing 0.2000M of acid,HA and 0.1500M of its conjugate base, A-, has pH of 3.35. What is the pH after 0.0015mol of NaOH is added to 0.5000L of this solution?

Answer:
Use the Henderson (buffer) equation with the 0.20 M, 0.15 M, and pH to find the pKa of the acid.

If you have 0.5 L, then you actually 0.10 mole of the acid and 0.0750 mole of the conj. base. So when you add the 0.0015 mol of NaOH, you decrease the moles of acid to 0.0985 moles, and you increase the moles of conj. base to 0.0765 moles. So your new concentration of acid is 0.197 M and your new concentration of base is 0.153 M.

Use the pKa that you found at the start and the new concentrations into the buffer equation to get the new pH.

These changes seem rather slight. Are you sure it wasn't 0.015 mol of NaOH that was added?

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